1. Field of the Invention
The present invention relates to substrate holders, especially those for holding thin, substantially planar substrates. More particularly, the invention relates to such substrate holders as used in lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table provided with first holding means for holding a substrate of a first type; and
a projection system for projecting the patterned beam onto a target portion of the substrate.
2. Description of the Related Art
The term xe2x80x9cpatterning meansxe2x80x9d or xe2x80x9cmaskxe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensured that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from United States Patents U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in United States Patent U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic apparatus, the substrate table is generally designed to hold substrates that have standard sizes and shapes. Typically, the smallest substrate that can be held is a circular substrate with a diameter of 75 mm (3xe2x80x3). Substrate tables are generally designed so as to have holding means tailored to standard substrates; it is vital that the holding means be capable of holding the substrate immobile, and designing such means for a standard size/type of substrate allows the design to be optimized. Various other types of substrate supporting tablesxe2x80x94such as found in auxiliary apparatus, substrate handling components, substrate transport devices, etc.xe2x80x94are also generally designed to handle substrates of a standard size and shape.
However, there are occasions when it may be desirable to process a substrate that is either not of a standard size or not of a standard shape. For example, conventionally used SiC substrates generally have a diameter of 50 mm (2xe2x80x3), which is too small for most conventional substrate holding means. Also, substrates may have different shapes than the standard. Most substrates are fully round, or may have a limited flat portion (hereafter referred to as a xe2x80x9cflatxe2x80x9d) along part of their circumference; however square or rectangular substrates of various sizes may also be desirable (e.g. when dealing with ceramic substrates for magnetic head production). Moreover, it may be desirable to process irregular shapes, each of which may be unique (e.g. as a result of breakage of a larger substrate into irregular pieces). These shapes and sizes are relatively uncommon, and it is rarely economical to design and build a substrate table and handling components specifically tailored to such non-standard substrates.
An object of the present invention is to provide means by which non-standard substrates (substrates of a xe2x80x9csecond typexe2x80x9d) can be used in a lithographic projection apparatus having a substrate table designed to hold a particular size and shape of substrate (substrates of a xe2x80x9cfirst typexe2x80x9d).
According to the present invention, there is provided a lithographic projection apparatus as specified in the opening paragraph, characterized by an intermediate substrate holder which can be held by the first holding means and is provided with second holding means for holding a substrate of a second type.
According to a further aspect of the invention, there is provided an intermediate substrate holder for holding a substrate (of a second type), said holder comprising:
a substantially planar body having a first major surface;
a matrix of protrusions extending from said first major surface, for supporting the substrate;
barrier means provided on said first major surface for defining a vacuum space between that surface and a substrate resting against the said protrusions;
means for exposing the vacuum space to a vacuum.
The present invention is advantageous because an intermediate substrate holderxe2x80x94which can be held on the first holding means on a substrate table in the standard manner, and which itself can hold a (non-standard) substrate of the second type upon its own (second) holding meansxe2x80x94provides a means whereby such a (non-standard) substrate of the second type can be processed by the (standard) holding means of the first type on a conventional substrate table, without requiring substantial modification to the substrate table or the holding means thereupon. Ideally, the intermediate substrate holder, with the non-standard substrate attached, would be such that it could also be handled by other substrate handling apparatus, without such handling apparatus requiring modification. Thus, although extra software may be required to control the apparatus, the hardware of the apparatus would not require modification.
A particular advantage of this invention is that it allows the processing of broken substrates to be completed. Breakage is an unfortunate problem that can occur during the processing of substrates. Substrates typically contain many devices that are being simultaneously manufactured. If the substrate is broken, it is likely that some of the devices that are being manufactured on the substrate (i.e. those not on the fracture lines) will be undamaged. It is therefore desirable to be able to continue the processing of these undamaged devices, in view of the cost that has already been incurred in the manufacturing process up to that stage. Obviously, these pieces of substrate will no longer be of a standard shape or size, and designing a substrate holder for each one would not be sensible.
According to a further aspect of the invention, there is provided a method of manufacturing a device comprising the steps of:
(a) providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
(b) holding the substrate on a substrate table having first holding means for holding a substrate of a first type;
(c) providing a projection beam of radiation using a radiation system;
(d) using patterning means to endow the projection beam with a pattern in its cross-section;
(e) projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material,
characterized in that steps (a) and (b) comprise the steps of:
providing a substrate of a second type, having a layer of radiation-sensitive material, to an intermediate substrate holder that is capable of holding the substrate of the second type and can itself be held by the first holding means on the substrate table; and
providing the intermediate substrate holder, with the substrate of the second type held thereon, to said substrate table.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.